1. Field of the Invention
The present invention relates to a memory element exchanging circuit for a redundant memory, and more specifically to a memory element exchanging circuit for non-volatile semiconductor memory composed of the example floating-gate field effect transistor memory elements.
2. Description of Related Art
As well known, a floating-gate avalanche-injection metal-oxide-semiconductor field effect transistor (called "FAMOS" hereinafter) comprise a P-type substrate having N.sup.+ -diffusion source and drain regions formed on a surface of the substrate separately from each other so as to form a channel region between the source and the drain. A floating gate is formed on the channel region through an insulating layer and a control gate is formed on the floating gate through another insulating layer.
In the above mentioned FAMOS, when the floating gate is an electrically neutralized condition, the FAMOS will be rendered conductive with application of a relatively low control gate voltage, for example 2 volts. Further, a high voltage, for example 20, volts is applied between the control gate and the drain, electrons will be injected to the insulated floatig gate, so that a threshold voltage of the memory element to the control gate will be elevated. Namely, in the case that the insulated floating gate hold the injected electrons, the FAMOS cannot be rendered conductive unless a high control gate voltage such as 8 volts is applied.
Accordingly, when the floating gate is in an electrically neutralized condition (called a "non-written condition" hereinafter), the FAMOS will be rendered conductive with application of a low control gate voltage. On the other hand, if the electrons are injected to the insulated floating gate (called a "written condition" hereinafter), since the FAMOS has an elevated threshold voltage as mentioned above, the FAMOS cannot be rendered conductive unless a high control gate voltage such as 8 volts is applied. Namely, the FAMOS is no longer rendered conductive with a low control gate voltage. Thus, the FAMOS can store a binary data of "0" or "1" by utilizing the change of the threshold voltage.
In addition, if the control gate, the source and the drain of the FAMOS in the written condition are grounded, and a ultraviolet light is irradiated to the surface of the FAMOS, the electrons held in the floating gate are excited and then discharged via the control gate or the substrate. As a result, the threshold voltage will be decreased and the FAMOS will be returned to the non-written condition. Namely, it becomes possible to write a new data to the FAMOS by executing the above mentioned writing operation.
As mentioned above, the FAMOSs can permanently store a binary data and can rewrite the stored data after erasing by the ultraviolet light irradiation. Therefore, the FAMOSs are widely used to constitute a non-volatile semiconductor memory. Namely, some of non-volatile semiconductor memories are composed of a number of FAMOSs arranged in the form of a matrix so as to form a memory array.
On the other hand, a large capacity of non-volatile semiconductor memories are demanded at present. However, if the capacity of the semiconductor memories is increased, it will become difficult to manufacture all memory elements included in the semiconductor memory with no defect. Therefore, it is a greatly way to provide additional memory elements (called "redundant memory element" hereinafter), and to use the redundant memory elements in place of a defective memory elements.
For replacement or exchange between the defective memory element and the redundant memory element, there has been provided a memory element exchange control circuit adapted to store an address of a defective memory element (which address will be called a "defective address" hereinafter) and to selectg the redundant memory element when the defective address is accessed.
Hitherto, the memory element exchange control circuit has include a FAMOS for storing the defective address. But, if the FAMOS storing the defective address had been the same as those of the non-volatile memory, when the memory is irradiated with a ultraviolet light for erase, the defective address would be erased together with the data stored in the non-volatile memory to be rewritten. Therefore, in order to protect the content of the FAMOS storing the defective address, the FAMOS of the memory element exchange control circuit is isolated from the ultraviolet light for erase as far as possible. For example, the FAMOS of the memory element exchange control circuit has been covered with a ultraviolet light interception aluminum overcoating connected to the source region. This is effective to some extent. But, it is not possible to completely cover the gate signal line portion and the drain signal line portion of the FAMOS with the ultraviolet light interception aluminum overcoating, and therefore, the ultraviolet light will be injected through the gate signal line portion and the drain signal line portion of the FAMOS. As a result, with repeated ultraviolet light erase operations, the content of the FAMOS storing the defective address will be gradually lost.
This is a serious problem, because it has been a general practice to write the defective address only one time before the non-volatile memories is shipped and because a user, who would repeatedly erase and write the non-volatile memory, cannot generally rewrite the defective address to the memory element in the memory element exchange control circuit.